avcodec/utvideodec: Set B for the width= 1 case in restore_median_planar_il()

Fixes: use of uninitialized memory
Fixes: 439878388/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_UTVIDEO_DEC_fuzzer-5635866203848704

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 59db32b433ea9e7766ec7fac994860ed15d7ed7d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/utvideodec.c

@@ -456,7 +456,7 @@ static void restore_median_planar_il(UtvideoContext *c, uint8_t *src, ptrdiff_t
         // second line - first element has top prediction, the rest uses median
         C        = bsrc[-stride2];
         bsrc[0] += C;
-        A        = bsrc[0];
+        A = B    = bsrc[0];
         for (i = 1; i < FFMIN(width, 16); i++) { /* scalar loop (DSP need align 16) */
             B        = bsrc[i - stride2];
             bsrc[i] += mid_pred(A, B, (uint8_t)(A + B - C));